This invention relates generally to drive mechanisms and more particularly to a power assist device for a manually powered, crank-operated drive mechanism such as those employed in bicycles, tricycles and the like.
The standard drive mechanism for a manually powered vehicle such as a bicycle or tricycle includes a shaft having crank arms extending from opposing ends of the shaft, and pedals attached directly or by a linkage to the ends of the crank arms for transmitting power from the vehicle rider's legs and feet to the crank arms for turning the shaft about its long axis. In an ordinary bicycle, the crank arms are driven through a power stroke by extension of first one leg of the rider and then the other. A chief difficulty in operating such a drive mechanism is driving the crank arms past the top dead center position, that is, the position of the crank arms at the maximum extension of a first leg when a second leg first begins to extend. At and near the top dead center position, the largest component of the force applied by the second leg, beginning its power stroke, is directed down the crank arm through the axis of rotation of the shaft so that little torque is applied to turn the shaft. Thus, the rider must supply a large amount of force against the pedals to produce only a little torque. In order to drive the crank arms past the top dead center position, riders typically stand up on the pedals or otherwise change body position so that a greater component of the force applied to the pedals is directed perpendicular to the crank arm and at a distance from the axis of rotation of the shaft. The problem is particularly acute when the bicycle or tricycle is powered with a reciprocating motion of the pedals, because the rider cannot effectively change the top dead center position by changing body position.
I have previously designed drive mechanisms which facilitate the application of torque to the shaft at and near the top dead center position. Reference is made to U.S. Pat. Nos. 3,779,099, 3,906,807, 4,029,334 and 4,159,652, showing these designs. Generally speaking, my prior designs either use alternative links for transmitting force from the pedals to the shaft such that a substantial component of force applied always lies along a line at a distance from the axis of rotation of the shaft, or allowed one crank arm to be accelerated through the top dead center area (i.e., such that the crank arms were separated by less than 180 degrees) during the power stroke of the opposite crank arm. Although these designs make pedaling easier and smoother by effectively eliminating the top dead center position, they require considerable modification to existing bicycles and tricycles.